Cecilia Burman has always had a problem with faces. As a child, she struggled to pick out her own face in school photos, and she is hard-pressed today to describe her mother's features. Over the years she has offended countless friends, passing them on neighborhood streets or in office hallways like strangers. "People think I'm just snobby," says Burman, 38, a computer consultant in Stockholm. "It makes me really, really sad to lose new friends because they think I couldn't bother to say hello."

There's a name for Burman's condition: prosopagnosia or, more informally, face blindness. The disorder was thought to be exceedingly rare and mainly a result of brain injury. Until a few years ago, there were perhaps 100 documented cases, says Ken Nakayama, a professor of psychology at Harvard. But last month a team of German researchers took the first stab at charting its prevalence, and the results, published in the American Journal of Medical Genetics, were remarkable. The new study showed that prosopagnosia (from the Greek prosopon for face and agnosia for ignorance) is highly heritable and surprisingly common, afflicting, in some form, about 1 in 50 people--more than 5 million in the U.S. alone. "That's huge," says Dr. Thomas Grüter of the Institute of Human Genetics in Münster, an author of the paper and himself a prosopagnosic. "It was a real surprise." (See ways you can live for 100 years.)

Within that group of sufferers, however, the condition varies widely. For the vast majority, the problem is not so much about detecting a face--prosopagnosics can see eyes, noses and mouths as clearly as anyone else--as it is about recognizing the same set of features when seeing them again. It's a disability that complicates everything from following a movie plot to picking a perp out of a lineup. While mild prosopagnosics can train themselves to memorize a limited number of faces (it's said to be like learning to distinguish one stone from another), others grapple with identifying family members and, in extreme cases, their own face. Gaylen Howard, 40, a homemaker in Boulder, Colo., says that when she's standing in front of a mirror in a crowded restroom, she makes a funny face so that, as she puts it, "I can tell which one is me."

Most prosopagnosics learn to cope early on. They distinguish people based on cues like hairstyle, voice, gait or body shape. They avoid places where they could unexpectedly run into someone they know. They pretend to be lost in thought while walking down the street. They act friendly to everyone--or to no one. In short, they become expert at masking their dysfunction. "This is probably why [the disorder] went unnoticed for so long," says Grüter.

In the new study, Grüter and his colleagues surveyed 689 local high school and medical students and diagnosed the disorder in 17. Further interviews with the families of 14 of these subjects revealed that each had at least one close relative with the same problem. Judging from the patterns of inheritance, Grüter speculates that prosopagnosia may be caused by a defect in a single, dominant gene, so that if one parent has it, each child has a 50% chance of inheriting it.